What is the main Conductor Materials that Commonly used in Electrical Lines?

What is the Commonly Used Conductor Materials for Electrical Energy Transmission Line Network?

The conductor is one of the important items as most of the capital outlay is invested in it. Therefore, the proper choice of material and size of the conductor is of considerable importance. The conductor material used for the transmission and distribution of electric power should have the following properties :

1. high electrical conductivity.
2. high tensile strength in order to withstand mechanical stresses.
3. low cost so that it can be used for long distances.
4. low specific gravity so that weight per unit volume is small.

All the above requirements are not found in a single material. Therefore, while selecting a conductor material for a particular case, a compromise is made between the cost and the required electrical and mechanical properties.

Commonly used conductor materials in electrical transmission line

The most commonly used conductor materials for overhead lines are copper, aluminum, steel-cored aluminum, galvanized steel, and cadmium copper. The choice of a particular material will depend upon the cost, the required electrical and mechanical properties, and the local conditions.

All conductors used for overhead lines are preferably stranded in order to increase flexibility.

In stranded conductors, there is generally one central wire, and round this, successive layers of wires containing 6, 12, 18, 24 ...... wires. Thus, if there are n layers, the total number of individual wires is 3n(n + 1) + 1. In the manufacture of stranded conductors, the consecutive layers of wires are twisted or spiraled in opposite directions so that layers are bound together.

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Conductor Stranding Perspective of Power Cable Construction 

1. Copper. 

Copper is an ideal material for overhead lines owing to its high electrical conductivity and greater tensile strength. It is always used in the hard drawn from as a stranded conductor. Although hard drawing decreases the electrical conductivity slightly yet it increases the tensile strength considerably.

Copper has a high current density i.e., the current carrying capacity of copper per unit of X-sectional area is quite large. This leads to two advantages. Firstly, a smaller X-sectional area of the conductor is required and secondly, the area offered by the conductor to wind loads is reduced. Moreover, this metal is quite homogeneous, durable, and has high scrap value.

There is hardly any doubt that copper is an ideal material for the transmission and distribution of electric power. However, due to its higher cost and non-availability, it is rarely used for these purposes. Nowadays the trend is to use aluminium in place of copper.

2. Aluminium. 

Aluminium is cheap and light as compared to copper but it has much smaller conductivity and tensile strength. The relative comparison of the two materials is briefed below :

(i) The conductivity of aluminum is 60% that of copper. The smaller conductivity of aluminum means that for any particular transmission efficiency, the X-sectional area of the conductor must be larger in aluminum than in copper. For the same resistance, the diameter of the aluminum conductor is about 1·26 times the diameter of the copper conductor.

The increased X-section of aluminum exposes a greater surface to wind pressure and, therefore, supporting towers must be designed for greater transverse strength. This often requires the use of higher towers with the consequence of greater sag.

(ii) The specific gravity of aluminum (2·71 gm/cc) is lower than that of copper (8·9 gm/cc). Therefore, an aluminum conductor has almost one-half the weight of an equivalent copper conductor. For this reason, the supporting structures for aluminum need not be made as strong as that of copper conductors.

(iii) Aluminium conductor being light, is liable to greater swings and hence larger cross-arms are required.

(iv) Due to the lower tensile strength and higher coefficient of linear expansion of aluminum, the sag is greater in aluminum conductors.

Considering the combined properties of cost, conductivity, tensile strength, weight, etc., aluminum has an edge over copper. 

Therefore, it is widely used as a conductor material. It is particularly profitable to use aluminum for heavy-current transmission where the conductor size is large and its cost forms a major proportion of the total cost of the complete installation.

3. Steel-cored aluminum. 

Due to low tensile strength, aluminum conductors produce greater sag. This prohibits their use for larger spans and makes them unsuitable for long-distance transmission.

In order to increase the tensile strength, the aluminum conductor is reinforced with a core of galvanized steel wires. The composite conductor thus obtained is known as steel-cored aluminum and is abbreviated as A.C.S.R. (aluminum conductor steel reinforced).

The steel-cored aluminum conductor consists of the central core of †galvanised steel wires surrounded by a number of aluminum strands. Usually, the diameter of both steel and aluminum wires is the same. 

The X-section of the two metals is generally in the ratio of 1: 6 but can be modified to 1: 4 in order to get more tensile strength for the conductor. Fig. 8.1 shows steel cored aluminum conductor having one steel wire surrounded by six wires of aluminum. 

The result of this composite conductor is that the steel core takes a greater percentage of mechanical strength while aluminum strands carry the bulk of the current. The steel cored aluminum conductors have the following advantages :

(i) The reinforcement with steel increases the tensile strength but at the same time keeps the composite conductor light. Therefore, steel cored aluminum conductors will produce smaller sag and hence longer spans can be used.

(ii) Due to the smaller sag with steel-cored aluminum conductors, towers of smaller heights can be used.

4. Galvanized steel. 

Steel has very high tensile strength. Therefore, galvanized steel conductors can be used for extremely long spans or for short line sections exposed to abnormally high stresses due to climatic conditions. 

They have been found very suitable in rural areas where cheapness is the main consideration. 

Due to the poor conductivity and high resistance of steel, such conductors are not suitable for transmitting considerable power over a long distance. However, they can be used to advantage for transmitting a small power over a small distance where the size of the copper conductor desirable from economic considerations would be too small and thus unsuitable for use because of poor mechanical strength.

5. Cadmium copper. 

The conductor material now employed in certain cases is copper alloyed with cadmium. An addition of 1% or 2% cadmium to copper increases the tensile strength by about 50% and the conductivity is only reduced by 15% below that of pure copper. Therefore, cadmium copper conductors can be useful for exceptionally long spans. However, due to the high cost of cadmium, such conductors will be economical only for lines of small X-section i.e., where the cost of conductor material is comparatively small compared with the cost of supports.